Process for making aminopolycarboxylic acid chelates of iron

ABSTRACT

A process for making Fe-EDTA chelate comprising adding iron oxide at a ratio of &lt;1 mole iron/mole EDTA to a mixture of NH 4  OH and EDTA wherein NH 3  /EDTA mole ratio is from 1.05 to 1.5, heating until reaction is complete, cooling to about 60° C. and adding sufficient NH 3  to dissolve and maintain Fe-EDTA chelate in solution, cooling to room temperature and oxidizing Fe ++  to Fe +++ . The process provides (1) a minimum of foaming and sludge formation during the reaction, (2) rapid dissolution of iron oxide and (3) a chelate product substantially completely in the ferric state.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of our copending application Ser. No.184,848, filed Sept. 8, 1980 now U.S. Pat. No. 4,364,871.

BACKGROUND OF THE INVENTION

Iron chelates of aminopolycarboxylic acids, e.g.ethylenediaminetetraacetic acid, are useful in numerous applications.Their aqueous solutions are employed in photographic processes, asdescribed in U.S. Pat. No. 3,293,036, and for application toiron-deficient soils to provide that element to plants grown therein.Other uses include the removal of H₂ S from gas streams and as acatalyst in aqueous system processes.

In U.S. Pat. No. 3,767,689 a method for preparing aqueous solutions ofiron chelates of aminopolycarboxylic acids is described. The methodinvolves heating iron oxide and the aminopolycarboxylic acid in anaqueous medium and subsequently neutralizing with a base. The parametersemployed in that process are: heating to a temperature of from 80° C. to102° C.; a reaction period of five to 20 minutes or up to several hourswith more difficultly soluble forms of iron oxide; employing about 10%excess of the aminopolycarboxylic acid over that stoichiometricallyrequired to react with the iron oxide; and employing a ratio of 0.1 to 1mole NH₄ OH per mole of aminopolycarboxylic acid (preferably 0.8 mole toform the iron-ammonium chelate and then about 1.5 moles additional tobring pH to 7 and insure neutralization of Fe⁺⁺⁺ chelate).

In the above patented method several problems are encountered: (1)excessive foaming and sludge formation is observed during the initialreaction of the aminopolycarboxylic acid with the iron oxide andammonium hydroxide upon heating; (2) the end product contains about 20%of the iron in the ferrous (Fe⁺⁺) form; (3) the attempted oxidation ofthe ferrous iron to ferric by bubbling air through the reaction mixtureduring the reaction at elevated temperatures apparently decomposes theaminopolycarboxylic acid, thus reducing the amount of total chelantavailable to form the iron-ammonium chelate.

SUMMARY OF THE INVENTION

The present invention is a process for the preparation of aniron-aminopolycarboxylic acid chelate, totally in the ferric state. Theprocedure results in minimal foaming during the reaction, rapiddissolution of the iron oxide, and the absence of any sludge formation.

Foaming and sludge formation are substantially reduced by employing amole ratio of NH₃ /aminopolycarboxylic acid of from slightly greaterthan 1 to about 1.5 during the initial dissolution of the iron oxide.When the iron oxide is completely reacted with the chelant, the mixtureis cooled and sufficient ammonia introduced to dissolve and maintain theiron chelate in solution. Finally, the reaction mixture is cooled toroom temperature and contacted with oxygen (air) to convert anyremaining ferrous iron to ferric.

DETAILED DESCRIPTION OF THE INVENTION

The following description shows examples to illustrate the invention andcomparative examples to illustrate the art.

EXAMPLE 1

At room temperature with continuous agitation, a 10-gallon reactor wascharged with 9530 milliliters distilled water and 13,474 g. (46.14moles) of ethylenediaminetetraacetic acid (EDTA). Thirty percent aqueousammonia, 3760 ml. (59.9 moles NH₃) was added to the acid slurry. At thispoint the theoretical NH₃ /EDTA molar ratio of 1.3 was verified byanalysis. With continued stirring, 3302.5 g. iron oxide (Fe₃ O₄) wereadded. This established an Fe-EDTA balance in excess of stoichiometricamounts (3% excess EDTA, by weight, based on the total weight of thesystem). The reactor contents were heated at 100° C. for 90 minutes withminimal foaming (<5% volume increase), rapid iron oxide dissolution, andno sludge formation. The reaction product was cooled to about 60° C. andadditional aqueous ammonia was added to adjust the pH to 8.0. A cleardeep reddish-brown solution resulted. Analysis of the product at thispoint showed 7.2% by weight total iron [2.0% ferrous ion asferrous-EDTA, the remainder (5.2%) as ferric-EDTA] and 1.8% free(non-chelated) EDTA. This value (1.8% free EDTA) based on total weightof the system, when normalized to initial concentrations prior to thesecond ammonia addition, equates to 2.0%. A decrease in the amount offree EDTA present in the reaction mixture (from 3% initially to 2%)resulted from degradation of the EDTA during the heating cycle. Thereaction mixture was purged with air at room temperature to oxidize theferrous ion. Typical ferrous ion oxidation rate data is given in TableI. The resulting iron chelate solution was diluted with distilled waterto 7% by weight total iron. Analysis showed it contained less than 0.07%ferrous iron.

                  TABLE I                                                         ______________________________________                                        Ferrous Ion Oxidation Data                                                    Time, Hr.     % Ferrous Ion                                                   ______________________________________                                        0             2.00                                                            1.00          1.62                                                            2.00          1.57                                                            3.25          1.10                                                            4.00          0.80                                                            4.75          0.45                                                            7.83          0.07                                                            ______________________________________                                    

The rate of EDTA degradation can be reduced by use of a purge to removedissolved oxygen from the reaction system. The following example showspreparation of iron-EDTA chelate as described in Example 1 except that anitrogen purge was employed.

EXAMPLE 2 (N₂ purge)

To 511 g. distilled water at room temperature was added 755.5 g. (2.59moles) EDTA. This mixture ammoniated with 200 milliliters (3.18 moles)of 30% by weight aqueous ammonia. With continuous stirring, 196.7 g. ofiron oxide (Fe₃ O₄) were added. The mixture was heated at 103.5° C. for90 minutes. The reaction mixture was purged with nitrogen during theheat cycle. Foaming was minimal, accounting for <5% volume increase.Samples were taken periodically and analyzed to determine iron-EDTAmolar balance. The reaction mixture was cooled to about 60° C. andammoniated to pH 8. Air was purged through the mixture at roomtemperature to oxidize ferrous ion (2.4% by analysis). Upon oxidation ofthe ferrous ion, the mixture was diluted with distilled water to a 7%iron product.

Another preparation, run concurrently, was made according to the methodof Example 1, using the same amounts of reactants, temperatures andtimes employed in Example 2, but without the nitrogen purge.

EXAMPLE 3 (absence of N₂ purge)

To a mixture of 756 g. (2.59 moles) of ethylenediaminetetraacetic acidand 511 g. distilled water at room temperature there was added 200milliliters (3.18 moles) of 30% aqueous ammonia. With continuousagitation, 200 g. of iron oxide (Fe₃ O₄) were added. The mixture washeated at 103.5° C. for 90 minutes. Foaming produced less than 5% volumeincrease. Samples were taken periodically and analyzed for Fe-EDTA molarbalance to monitor degradation of the EDTA. The reaction mixture wascooled to about 60° C. and ammoniated to pH 8. Air was purged throughthe reaction mixture at room temperature to oxidize the ferrous ion. Theresulting oxidized mixture was diluted with distilled water to a 7% ironproduct.

Table II shows the relative rates of degradation determined for Examples2 and 3.

                  TABLE II                                                        ______________________________________                                        Wt. % Free EDTA (based on total                                               system weight)                                                                Time, Min. Ex. 2 (N.sub.2 purge)                                                                          Ex. 3                                             ______________________________________                                        10         2.5              2.7                                               15         2.4              1.4                                               30         1.0              0.3 excess Fe                                     45         0.8              0.4 excess Fe                                     65         0.5              0.3 excess Fe                                     ______________________________________                                    

While examples are employed using iron oxide (Fe₃ O₄) and EDTA asillustrative of the invention, other aminopolycarboxylic acids can beemployed as the chelant moiety. Thus, representative acids arenitrilotriacetic acid, diethylenetriaminepentaacetic acid,N-hydroxyethylethylenediaminetriacetic acid,ethylenediaminetetrapropionic acid, nitrilotripropionic acid,N-hydroxyethyliminodiacetic acid, and triethylenetetraminehexaaceticacid.

The following experiment shows the effect of the ratio of ammoniumhydroxide (NH₄ OH) to chelant on foaming during the reaction of ironoxide with a chelating agent in the presence of NH₃ in aqueous solution.The pH of various ratios of NH₄ OH/chelant was also determined.

EXAMPLE 4

Into a 1-liter, tall-form graduated beaker, 71/2" high, was placed aquantity of ethylenediaminetetraacetic acid (EDTA) in water. The mixturewas stirred at room temperature with a magnetic stirrer and concentratedammonium hydroxide in a given amount was added slowly while stirring.Iron oxide was added and stirring continued until a uniform slurry wasobtained. The liquid level of each run was the same, i.e. at the 500milliliter mark on the beaker, or ˜31/2" from the bottom of the beaker.

The initial stirring velocity was set and maintained unchanged whilestirring was continued while heating to reaction temperature. This wasaccomplished by using a hot plate on which the beaker was heated untilthe contents reached a temperature of 100°-105° C. When this temperaturewas reached the initial foam height above the liquid level was measuredand the physical characteristics of the reaction mixture noted. Stirringwas then increased by setting the velocity at a new constant level whichwas the same for all runs. The height of the foam and/or control offoaming was again noted. Normally, an increased stirring rate isexpected to reduce or control the foaming. Table III, following, showsthe amounts of materials and ratios for each run.

                  TABLE III                                                       ______________________________________                                                 Run No.                                                              Components 1       2       3     4     5                                      ______________________________________                                        NH.sub.4 OH/EDTA                                                                         0.85    1.00    1.05  1.10  1.20                                   (Mol Ratio)                                                                   EDTA (g)   272.6   272.3   272.4 272.7 272.4                                  Water (ml) 197     188     185   182   176                                    Conc. NH.sub.4 OH                                                                        51      60      63    66    72                                     (ml)                                                                          Iron oxide (g)                                                                           70.3    70.5    70.8  72.4  72.3                                   (Fe.sub.3 O.sub.4)                                                            ______________________________________                                    

Table IV below shows the foam data and slurry characteristics for theruns in Table I.

                  TABLE IV                                                        ______________________________________                                                Inital Foam                                                                              Foam Height                                                Run     Height Upon                                                                              With Increased                                                                              Slurry                                       No.     Reaction   Stirring      Character                                    ______________________________________                                        1       >4"*       >4"*          Thick,                                                                        viscous                                      2       >4"*       21/2"         Some slurry,                                                                  less viscous                                                                  than Run 1                                   3       23/4"      1"            Some slurry,                                                                  less viscous                                                                  than Run 2                                   4       <1/16"     0             Fluid                                        5       0          0             Fluid                                        ______________________________________                                         *Foamed out the top of the beaker                                        

The data in Table IV above show that there is a definite improvement inability to run the process because of decreased foaming when mol ratiosof NH₃ /EDTA of 1.05 and above are employed.

The following Table V shows the pH measured for an initial NH₄ OH/EDTAmixture containing ratios over the same range as in Table III. Severalratios outside the range are also given.

                  TABLE V                                                         ______________________________________                                               NH.sub.3 /EDTA                                                                Molar Ratio                                                                            pH                                                            ______________________________________                                               0.7      4.35                                                                 0.85     4.38                                                                 1.00     4.41                                                                 1.05     4.43                                                                 1.10     4.44                                                                 1.25     4.48                                                                 1.50     4.55                                                          ______________________________________                                    

The conclusion that can be drawn from Table V is that the range claimedin the present invention could not have been predicted using pH as aguide since there is no sharp break in pH, but the increase is gradualand continuous from below and through the range claimed.

The following comparative examples were conducted in accordance with theteachings of U.S. Pat. No. 3,767,689.

COMPARATIVE EXAMPLE A

Ethylenediaminetetraacetic acid (754 g., 2.58 moles) was added to 512 g.distilled water. One hundred thirty milliliters (2.06 moles) of 30%aqueous ammonia were added to the acid slurry to achieve an ammonia/EDTAmolar ratio of 0.8. With continuous agitation, 198.4 g. of iron oxide(Fe₃ O₄) were added. The reaction mixture was heated to 100° C. at whichpoint it began to foam exceedingly resulting in an 80% increase involume. The foaming was followed by heavy sludge formation which wasdifficult to stir. Any sampling of the system at this point would havebeen futile due to solid formation and non-homogeneity of the mixture.

Following the reaction (45 minute heat cycle), additional 30% aqueousammonia was added, after cooling to about 60° C., to neutralize theproduct and raise the pH of the solution to 7.5. The mixture was dilutedto form a 7% iron product. Analysis of the final product showed 1.5%ferrous iron (or ˜20% of the total iron was in the ferrous state).

In an attempt to prepare a totally ferric-EDTA chelate, a run was madeutilizing an air purge during the heat cycle to oxidize ferrous ion. Seebelow.

COMPARATIVE EXAMPLE B

To a mixture of 512 g. distilled water and 758 g. (2.60 moles) EDTA atroom temperature, 200 ml. (3.18 moles NH₃) of 30% aqueous ammonia wereadded. With continuous stirring, 194 g. of iron oxide (Fe₃ O₄) wereadded.

The reactor contents were heated at 100.5° C. for 90 minutes with an airpurge. Following the heat cycle, the product was cooled to about 60° C.,then ammoniated to pH 8.0 and diluted with distilled water. The finalproduct contained 7% total iron (1.5% ferrous ion, 5.5% ferric ion).

Other oxides of iron, e.g. FeO and Fe₂ O₃, are also useful in preparingthe ferric chelate, recognizing that the ferrous ion must be completelyconverted to the ferric when using FeO, which would perhaps lengthen thetotal process.

Temperatures operable in the present invention are within the range of85° to 105° C. with a preferred range being from about 95° to 100° C.

Lower temperatures result in slow and/or incomplete iron oxidedissolution; higher temperatures increase the rate ofaminopolycarboxylic acid degradation, both of which results areundesirable.

Reaction times are determined by the time required to completelydissolve the iron oxide. Reaction times as short as 15 minutes have beenutilized resulting in small amounts of unreacted iron oxide. Heatingtimes in excess of that required for complete iron oxide dissolutionallow for aminopolycarboxylic acid degradation.

The initial mole ratios of NH₃ to chelant, e.g. EDTA, are from 1.05 toabout 1.5 with the preferred range being from about 1.1 to about 1.3.Molar ratios of 1.0 or less result in extensive foaming and sludgingwhile values greater than 1.5 result in very slow and/or incomplete ironoxide dissolution.

In the process of the present invention, it is necessary to use anexcess of chelant based on a stoichiometric 1:1 molar ratio of iron tochelant. The iron may be present in an amount within the range of fromabout 0.8 to about 0.96 mole per mole of chelant, but is preferablyabout 0.90-0.94. In terms of weight percent of the total system 2-9%excess may be employed, but a preferred excess of chelant is from about2-4%. Free EDTA in excess of 4% may be used but is not economicallyattractive. Smaller amounts of free EDTA (i.e., less than 2%) can beutilized provided some excess EDTA remains following the reaction forproduct stability.

The ammonia is normally provided as concentrated NH₄ OH (about 30% NH₃),but anhydrous ammonia can be employed.

Thus, the improved process of the present invention provides (1) minimalfoaming and no sludge, (2) a product in which substantially all the ironis in the ferric state and (3) a reduction in the rate of decompositionof the chelant during the reaction.

The steps of the process of forming the iron chelate which it is desiredto claim are: (1) providing a mixture (slurry) of NH₄ OH andaminopolycarboxylic acid chelant in an aqueous system wherein (2) theNH₃ /chelant ratio is greater than 1.0 but no more than about 1.5, (3)heating said mixture to a temperature of from about 85° to about 105° C.while (4) introducing less than a stoichiometric amount of iron oxide toform an iron-ammonium-chelant product, (5) heating said mixture untilthe reaction is complete, optionally in the substantial absence ofoxygen, (6) cooling to a temperature of from about 45°-75° C. and addingsufficient ammonia to dissolve and to maintain the iron chelate insolution, and (7) cooling said reaction mixture to room temperature andcontacting with an oxidizing agent for a sufficient time to ensure thatthe iron is substantially all in the ferric state.

We claim:
 1. A process for producing a ferric-ammonium-chelate of anaminopolycarboxylic acid wherein an oxide of iron is reacted with anaminopolycarboxylic acid chelant in the presence of a base, whichcomprises: (1) providing a mixture in water of ammonia together withsaid chelant in a molar ratio of ammonia to chelant of at least 1.05 butnot more than 1.5, (2) adding to said mixture said oxide of iron at lessthan 1 mole of iron per mole of chelant, (3) heating said mixture to areaction temperature within the range of from about 85° to about 105°C., (4) maintaining said reaction temperature for a time sufficient tocomplete the reaction, (5) cooling said mixture to a temperature withinthe range of from about 45° to 80° C., (6) introducing ammonia to saidmixture in sufficient amount to dissolve and to maintain in solution theiron chelate so formed, (7) cooling said chelate solution to roomtemperature and (8) oxidizing any ferrous ion present in said chelatesolution to the ferric ion.
 2. The process of claim 1 wherein theaminopolycarboxylic acid chelant is ethylenediaminetetraacetic acid. 3.The process of claims 1 or 2 wherein the mole ratio of ammonia tochelant in step (1) is from about 1.2 to about 1.3.
 4. The process ofclaims 1 or 2 wherein the oxide of iron in step (2) is added at a molarratio of from about 0.8 to about 0.96 mole per mole of chelant.
 5. Theprocess of claims 1 or 2 wherein the temperature of reaction in step (3)is in the range of 95°-100° C.
 6. The process of claim 1 wherein theammonia is added in step (6) in an amount sufficient to provide a pH ofabout 8 to said solution.
 7. The process of claim 2 wherein theoxidation of step (8) is accomplished by contacting with anoxygen-containing gas.
 8. The process of claim 7 wherein theoxygen-containing gas is air.
 9. The process of claim 2 wherein theoxide of iron is Fe₃ O₄.